The overall health related objective of this project is to make available new varieties of hydroxyapatite (HA) that are optimized for different types of biocompatible implant situations. Thus, the medical practitioner will eventually have available a better implant for replacing structures as widely different as teeth and bone. One major goal of the research program is to develop low temperature hydrothermal synthesis methods for the preparation of "ideal" phase-pure nonstoichiometric hydroxyapatite powders, with controlled CO32- and HPO42- content and submicron, equiaxed, monodisperse and unagglomerated particles. These powders will be suitable for making implant materials by standard ceramic processing techniques. In order to control CO32- and HPO42- substitution, an aqueous thermodynamic model will be developed to predict the solution conditions needed to achieve the desired nonstiochoimetric HA composition. More specifically, the model will be used to predict hydrothermal precipitation of HA with controlled carbonate, phosphate, and hydroxyl content. Having mapped out aqueous solution variable space (T, pH, pCa, pCO3-2, and pElectrolytes) for preparation of phase-pure nonstiochoimetric HA, experimental validation of the model will be sought. Chemical and phase analysis will be used to characterize the products to assess the correlation between model and experiment. With a validated model, we will design experiments that focus on control of HA morphology and particle size for specific nonstoichiometric compositions of HA. A range of solution processing variables, as defined by our thermodynamic model, including concentration, pH, Ca/P and traces of additives such as fluoride ions will be examined to accomplish this goal. The results of this project will form the basis for future work on the fabrication of engineering materials for implant testing and manufacturing.